# Keep Air Conditioner On All Day, or Turn it Off When Not Home? (2-story apt)

by mirumirai
Tags: 2story, conditioner, home, turn
 P: 5 I haven't been able to find a satisfactory answer. What would save the most electricity? One of my roomates and some of the neighbors below us think that it is better to leave the air conditioner on all day. Normally, for just one house with one air conditioner, I would be very skeptical: I am more convinced by this explanation here: http://michaelbluejay.com/electricity/cooling.html My neighbor brings up an interesting point that I didn't consider: that we have two floors. He says, "Like it or not we all share the same house. If we turn off our air when we're not home, your air has to work twice as hard to stop our hot air from displacing it. Since we don't all live on the same schedule, we're going to end up causing each other's floors to work harder than they have to." How does this change the dynamic, with the 2 ACs out of sync? Ideally, we would have programmable thermostats, but we don't. In addition, there is the issue of moisture, and how long it takes the air conditioner to "warm up." This website (http://www.srpnet.com/energy/ask/air.aspx) says it takes an AC 7-10 min. to reach max efficiency and recommends leaving the AC on due to moisture. Now that it's Fall, AC doesn't matter as much, but I am very interested in finding out the truth. Edit Sorry! Wrong Forum! I made a new thread in General. Feel free to delete this one.
 P: 1,410 Leave it on. It takes more energy to have to constantly cool down all of the wall, floors, etc. every time you turn it on again after turning it off. My roommates and I found out the hard way one summer when we turned it off when we all went to work. We got smacked for over $100 more on our electric bill that month vs. when we just left it on the whole time.  PF Gold P: 7,120 Quick question, are the two apartments open to each other (ie. a common entry that air can move between)? Or are they fully separated?  P: 1,035 Keep Air Conditioner On All Day, or Turn it Off When Not Home? (2-story apt) You could always get some sort of wattmeter and compare the two cases under similar outside conditions. I don't think there can be any one blanket statement that covers this because there are just too many variables. What is the surrounding temperature, insulation, duration you are gone, do your windows face the sun, are your two homes really connected so that they share some work, or does the floor provide an adequate insulation... Mentor P: 3,005  Quote by QuarkCharmer You could always get some sort of wattmeter and compare the two cases under similar outside conditions. I don't think there can be any one blanket statement that covers this because there are just too many variables. What is the surrounding temperature, insulation, duration you are gone, do your windows face the sun, are your two homes really connected so that they share some work, or does the floor provide an adequate insulation... I agree, the variables make a definitive answer impossible. To the OP: how's the insulation?  HW Helper P: 2,155 I have to agree with QuarkCharmer that it depends on your situation. Leaving the AC on all the time could use more or less energy than turning it off when you're out, depending on how well insulated your house is and how long you're typically out for. (The page you linked to says "This means that throughout the day, your house has absorbed way more than one houseful of heat" but that's not automatically true; it depends on the rate of heat flow and the time over which it flows.)  P: 5 Thanks for all the replies! Our insulation doesn't seem that great. The two apartments are for the most part separate. There is one staircase connecting the two, but there are doors on both ends of the staircase. I think the separation is pretty good. Also, we are on the top apartment, so shouldn't hypothetically, we have more trouble cooling our apartment down/AC costs more for us, since heat rises? I'm happy to answer any more detail questions so we can get to the bottom of this.  P: 77 Ideally, you'll want the compressor running the most when the air outside is coolest. Problem, is, that's when you typically need your A/C the least. I have a programmable, and I set it to follow the daily average temperature. It may be cold at night, but I've a good blanket. I get it cold enough at night, when the air's coldest outside, that it rarely needs to kick in during the day until after the air turns cooler in the evening. That alone cut my heating and cooling bills in Florida by about 35%. I had a heat pump there, though. Here I just have an A/C with gas heat. I think the benefits of following the temps in the winter will be much less pronounced because it's not a heat pump, but there's still solar heating and nighttime heat loss, so there will still be some benefit. Basic programmables are cheap these days, as little as$15. In your situation, you should be them for all residents. You don't all have to agree on the same temp, but if you followed the same cycle, the bills would be more even.
 Mentor P: 11,997 Hrmm. I just responded to this identical post elsewhere....
 P: 1,550 One thing to consider is that perhaps you are over working your AC if you leave it on all day. At the apartment complex I work at they have AC in several of the rooms of the clubhouse. During the summer the residents always turn on the AC and never turn it off when they leave. And nearly every summer one of those AC units winds up dying. They just slowly over time lose their "coolness' until they are blowing nothing but warm air. These are wall/window units.
 Mentor P: 22,313 Sorry guys, this question really does have an easy answer: turn it off or raise the thermostat. The first link is correct about why. For the shared walls thing, you lose some of the benefit if you and your neighbor have different schedules, but not a lot of it. Regarding efficiency: yes, it is more efficient when run longer: which is when you get home and turn it on. Bonus: it is also more efficient when the room is warmer.
Mentor
P: 22,313
 Quote by gravenewworld Leave it on. It takes more energy to have to constantly cool down all of the wall, floors, etc. every time you turn it on again after turning it off.
That's not even wrong. Leaving it on means the house and walls are kept cooler, which takes more power because they absorb more heat.